(1) Field of the Invention
The present invention relates to a hybrid aircraft having a rotary wing and suitable for traveling long distances at high speed.
This advanced rotorcraft design seeks, at reasonable cost, to combine the effectiveness in vertical flight of a conventional helicopter with the high travel speed performance made possible by using propulsive propellers and modern engines.
(2) Description of Related Art
In order to understand clearly the object of the invention, it is appropriate to recall the main kinds of flying machine that correspond to aircraft and to rotorcraft.
The term “rotorcraft” designates any aircraft in which lift is provided in full or in part by at least one rotary wing. The rotary wing usually comprises at least one rotor of large diameter and of axis that is substantially vertical when the aircraft is standing on the ground.
The rotorcraft category includes several distinct types.
Firstly, there is the helicopter in which at least one main rotor under drive from an appropriate engine, serves to provide both lift and propulsion.
A helicopter may have two lift rotors providing the aircraft both with lift and with propulsion. The two rotors may be arranged one behind the other or they may be on the same axis.
There is also the autogyro, which is a rotorcraft in which the rotor does not receive power, but provides lift by autorotation under the effect of the forward speed of the aircraft.
There is also the gyrodyne, which is a rotorcraft that is intermediate between the helicopter and the autogyro, in which the rotor provides only lift. The rotor is normally driven by a power plant during stages of take-off, hovering or vertical flight, and landing, like a helicopter. A gyrodyne also has an additional propulsion system that is essentially different from the rotor assembly. In forward flight, the rotor continues to provide lift, but solely in autorotation mode, i.e. without power being transmitted to said rotor.
There is also the compound rotorcraft, which takes off and lands like a helicopter, while performing cruising flight like an autogyro.
Of those various rotorcraft formulae, the helicopter is the simplest, such that it is the most widespread in spite of the fact that the maximum forward speed of a helicopter is about 300 kilometers per hour (km/h) which is slow and less than the speed that can be envisaged with formulae of the compound or convertible types, that are technically more complex and more expensive.
Another novel formula is known as the “hybrid” helicopter and is described in document EP 2 148 814.
Document EP 2 105 378 describes an aircraft having a rotary wing, two propulsive propellers, and a stabilizer surface at the front of the fuselage.
The arrangement of propellers enables the sides of the aircraft fuselage to be left free, e.g. for loading and unloading passengers. In addition, those propellers are arranged at the rear of the aircraft, thus serving to minimize the amount of propeller-generated noise that is perceived by passengers.
It should also be observed that the aircraft does not have a tail boom or a vertical tail fin, thereby serving at least to minimize the phenomenon known as “tail shake”.
In another aspect, the stabilizer surface placed at the front of the aircraft serves at least to minimize the “attitude hump” phenomenon (from the French: “bosse d'assiette”).
A rotorcraft is also known that has two main rotors on a common axis acting together to provide the aircraft with lift and not requiring an anti-torque device.
Such a rotorcraft also includes a propeller situated at the rear end of the aircraft to provide the aircraft with propulsion in cruising flight, and a declutching system enabling the propulsive propeller to be driven or not.
That rotorcraft thus possesses a system for transmitting power to the main rotors that is relatively complex.
In addition, the propeller is potentially situated in the wake of the fuselage and of a main rotor, which can give rise to noise and to the phenomenon known as “tail shake”.
Furthermore, a rotorcraft is known having a main rotor that provides the aircraft with lift in hovering flight, with all or part of its lift in cruising flight, and also with some of its propulsion in cruising flight.
The rotorcraft also has a tail rotor that provides an anti-torque function and a function of controlling the aircraft in yaw, a fixed wing that provides additional lift in cruising flight, and a non-declutchable propeller situated at the rear end of the aircraft for contributing to propulsion.
It should be observed in particular that it is not possible in hovering flight to stop the propeller from rotating, that propeller then requiring power, even when providing no thrust.
Document U.S. Pat. No. 3,448,946 proposes a compound rotorcraft with a propulsive tail rotor and an anti-torque tail rotor.
Document U.S. Pat. No. 3,105,659 describes a two-mode aircraft operating both in helicopter mode and in autogyro mode.
Document US 2002/0011539 presents an aircraft having propulsive propellers that provide an anti-torque function.
Document CN 1 098 688 presents an aircraft having a differential mechanism.
Also known are the following documents: U.S. Pat. No. 2,665,859 A; US 2005/151001 A1; GB 215 366 A; GB 2 197 629 A; and GB 895 590 A.